When an explosive device detonates, high pressure and high temperature are created. In the chemical reaction, compounds break down to form various gases. The concentrated gases are under very high pressure and thus, they expand rapidly. The heat speeds up the individual gas particles, boosting the pressure even higher. In a high explosive, the gas pressure is strong enough to destroy structures and injure and kill people. If the t expands at a rate faster than the speed of sound, the fast expanding gas generates a powerful shock wave that causes the crashing of encountered objects. The pressure can also push pieces of solid material outward at great speed, causing them to hit people or structures with immense force.
The unique burning of explosives creates a sonic wave which velocity depends on the material in which the sonic wave is moving: the material density, elasticity and temperature. The magnitude of the sonic wave is not constant but decays at rate relative to L3, where L is the distance from the place of detonation, given by the equation:
      E    b    =                    K                  b          ⁢                                                    *      1              L      3      where E is the energy of the detonation after the explosion, b denotes the type of explosive, K is the explosion constant (for TNT, for example, K=1). The blast impact also depends on the shape of the explosive device. For example: a square explosive device creates a different wave than a conic explosive device or a bullet-like explosive device.
A variety of windows offer blast resistance and impact resistance solutions, most of which typically offer a single type of protection, namely bullet proof, blast resistant or impact resistant. But prior art blast resistant windows cannot withstand the detonation of an explosive device while in contact with the exterior pane of the window. Prior art blast resistant windows may withstand an explosion event that take place several meters away from the window and even tens and hundreds of meters from the window.
U.S. Pat. No. 3,624,238 is concerned with a bullet resistant structure of laminated character comprising outer faces or piles of safety glass with an intermediary ply formed by a polycarbonate and a resin.
U.S. Pat. No. 4,312,903 deals with an impact resistant double glazed structure and is concerned in particular with the thickness of the layers of the laminated window panes, and their chemical compositions. U.S. Pat. No. 6,333,085 discloses fixed double glazing window systems which offer improved protection against both blast and impact hazards, but will not withstand a contact blast.
Triple glazed windows are also known as described, for example in U.S. Pat. No. 5,553,440. Such windows may also be broken easily. U.S. Pat. No. 6,108,999 offers some improvement providing a window glazing unit which may be bullet-resistant, and which offers the advantages of being shatter-resistant but not blast resistant. U.S. Pat. No. '999 provides a window 10 illustrated in FIG. 1. Window 10 includes two glass sheets 30 and 32, and a shatter-resistant thermoplastic sheet 20 in between glass sheets 30 and 32. A U shaped part 40 supports thermoplastic sheet 20, thus thermoplastic sheet 20 is smaller in size than glass sheets 30 and 32. Being smaller in size further reduces the elasticity effect of thermoplastic sheet 20, which elasticity is not sufficient to absorb the blast impact of a bomb which detonates a few meters away.
There is therefore a need for a window pane system which can offer improved protection against blast and impact hazards, including contact blasts caused by a bomb or a shell, with or without hollow charge and bullets.
FIG. 2 illustrates the operational steps of a hollow charge device 60. In step 80 device 60 is in a state before detonation; in step 82 device 60 is in a state just after detonation; in step 84 device 60 is in a state well after detonation, where a liner 99 has started to form; in step 86 device 60 is in a state just after detonation. In step 86, the metal glazing 70 of the inner cone has completely converted into a liner 99 which moves forward at an accelerated speed. The shape of cone 92, that is the radius and head angle, determine the angle at which the shock wave will move forward.
To eliminate the effect of liner 99, the symmetry of device 60 and more particularly, the symmetry of inner cone 92 must be breached, which is done by slat armor 50 such as metal bars nets and the like (see FIG. 3). Slat armors are statistical barriers that are disposed in front of a window to be protected. When device 60 hits the net of slat armor 50 and the dent formed in device 60 cancels the symmetry of device 60 and thereby the effect of liner 99 is eliminated.